For many years, it has been known to employ composites comprising a heat resistant organic resin coating on a metal base, such as aluminum and stainless steel for cooking utensils. Pots and pans and other cooking utensils employing this composite enjoy non-sticking properties due to the resin coating which make them very easy to clean.
An improved composite for such use has also been employed wherein a glass or ceramic frit is bonded to the metal base over a portion of its surface area and then the glass frit and metal base are coated with the organic resin. This composite when used for cooking utensils provides the added improvement of rendering the organic resin coating wear resistant.
The aluminum alloy generally used for this purpose is the low magnesium Alloy AA 3003, which has a composition of up to 0.6% silicon, up to 0.7% iron, from 0.05 to 0.20% copper, from 1.0 to 1.5% manganese, balance essentially aluminum. However, this alloy has not been entirely satisfactory. Only simple shapes can be enameled because of distortion during this operation which is carried out at 1000.degree.F and takes about 15 minutes. The distortion generally occurs in the portion of the sidewall which is cold worked and reduced in gage during the drawing operation. An additional problem with Alloy 3003 is that after it is enameled, the material is fully annealed and very soft. Hence, it is susceptible to denting damage, crazing, chipping and spalling of the ceramic. In addition to the foregoing, the sidewall of an article such as a pot must have sufficient flexural strength when loaded so that when lifted by the handle it does not collapse. In order to comply with this requirement, Alloy 3003 must be either made thicker or cold worked in some way in order to increase the yield strength.
The above problem has been noted in the prior art as reported by U.S. Pat. No. 3,788,513, to Racz. Patentee notes that the preparation of procelain coated cookware from aluminum suffers from the thermal stability for high purity aluminum at porcelainizing temperatures, with the results that such material had to be employed in increased thicknesses to compensate for loss of strength. Further, patentee notes that certain aluminum alloys which exhibit favorable thermal properties in that they age-harden to acceptable strength levels after exposure to porcelainizing temperature levels, are nonetheless unacceptable in cookware applications as they contain quantities of magnesium which renders them incompatible with procelain enamels. From the above it may be concluded that an acceptable aluminum alloy for use in cookware applications should be one that is compatible with porcelain enamel and capable of retaining its strength subsequent to exposure to porcelainizing temperature. In other words, a suitable aluminum alloy would have to be a non-magnesium containing high temperature aluminum material.
In the above context, the art suggests very few materials meeting the above criteria of high temperature stability and reduced or non-existent magnesium content. Of the alloys listed and registered with the Aluminum Association, only one alloy, namely, Alloy 2219, possesses the requisite properties to meet the aforenoted criteria. This alloy is a known high temperature alloy whose primary utility resides in such applications as rocket nose cones and the like, and which possesses a magnesium content ranging up to a maximum of 0.02% by weight. Another aluminum base alloy possessing high temperature stability which contains no magnesium is disclosed in U.S. Pat. No. 3,386,820, to Jagaciak, assigned to the assignees of the present application.
Though the selection of either of the above alloys would appear theoretically correct, an additional characteristic unexpectedly found to be possessed by one of the alloys renders it alone suitable in cookware applications. This characteristic, referred to hereinafter as formability, relates to the ability of the alloy to undergo the deformation required to prepare articles such as pots and pans. As will be seen hereinafter, this property has not been previously recognized and investigated.
It is, therefore, highly desirable to provide a replacement alloy which overcomes this disadvantage, while still meeting the stringent requirements for this difficult composite. For example, a replacement alloy for use in this composite would have to retain the procelain enameling and the resin coating capability of AA 3003 and provide increased room temperature strength after exposure to the thermal treatment, while overcoming the aforenoted disadvantage and withstanding distortion during enameling. A further requirement is that the aluminum alloy must have sufficient formability to make the desired configuration and do so with minimum earing. An additional requirement is a fine grain size before forming since a coarse grain results in the esthetically objectionable phenomenon known as "orange peel".